1. Background of the Invention
The present invention relates to seismic survey acquisition equipment. In particular, the invention relates to seismic survey equipment assembly combinations, survey data management strategies, operating software for carrying out the management strategies, the logistics of equipment deployment, and operation of equipment.
2. Description of Related Art
In principle, a seismic survey represents a voluminous data set containing detailed information that may be analyzed to describe the earth's layered geology as indicated by seismic wave reflections from acoustic impedance discontinuities at the layer interfaces. The analysis is influenced by elastic wave propagation velocities respective to the differences in strata density or elasticity. A seismic event such as is caused by the ignition of buried explosives in a shallow borehole or by a vibratory mechanism placed at the earth's surface is launched into the earth at a precisely known location and time. Seismic wave reflections of this man-made seismic event are detected by a multiplicity of transducers characterized in the art as geophones. The geophones are distributed in an orderly grid over the area of interest. The location of each geophone array is precisely mapped relative to the location of the seismic event. As the seismic wave from the timed event travels out from the source, reflections from that original seismic wave are returned to the surface where they are detected by the geophones. The geophones respond to the receipt of a wave with a corresponding analog electrical signal. These analog signals are received by data acquisition modules that digitize the analog signal stream for retransmission to a central recording unit. Among the significant data digitized by a data acquisition module may be the amplitude or strength of the reflected wave and the exact time lapse between the moment the event occurred and the moment an analog value of the geophone array is translated to a digital value.
In a single survey, there may be thousands of geophone signal sources. Consequently, the data flow must be orderly and organized. For example, the data acquisition modules transmit geophone signal values in digital data groupings called packets. Each packet contains a predetermined number of digital data bits representing, among other things, the digital value of the analog signal amplitude at the time that a seismic wave or increment thereof was translated to the digital value. The acquisition modules are programmed to transmit a data packet respective to a set of geophone channels at a predetermined bit rate. The variable data in a data packet represents an instantaneous snapshot of the analog signal flow from each geophone channel. There may be numerous individual geophone units transmitting respective analog signals to the data acquisition module on the same geophone signal channel.
Managing an orderly flow of this massive quantity of data to a central recording unit, often in a field survey truck, requires a plurality of digital signal processing devices. The data acquisition modules convert the geophone analog data to digital data and transmit the digital data packets along receiver lines or radio transmission channels. There may be numerous data acquisition modules transmitting respective data packets along a single receiver line. Among the functions of each data acquisition module is data packet transmission timing respective to the flow of data packets from other data acquisition modules transmitting respective data packets along the same receiver line. Typically, two or more receiver lines connect with base line units that further coordinate the data packet flow of numerous additional base line units into a base transmission line for receipt by a central recording unit.
Seismic surveying is often carried out under extremely inhospitable conditions of heat or cold, tropics or arctic, land or sea, desert or swamp. Regardless of the environment, the geophones must be positioned precisely relative to the seismic source event. Necessarily, manual placement of the geophones is normally required.
One of the many challenges facing seismic ground crews using cable connected systems is the initial decision of cable configuration(s). Data demands by geologists and investors are not always predictable. Seismic contractors must try to choose cable configurations that minimize weight for their workers in the field while keeping the number of line connectors to a minimum. However, prior art seismic systems are inflexibly designed as an integrated unit. If a remote data acquisition module is designed to operate in an 8-channel mode, a prior art system cannot readily be reconfigured to operate in a 6-channel mode notwithstanding that a particular survey task may be especially suited to the 6-channel mode. Prior art data acquisition modules are manufactured for a typical configuration with a fixed bit transmission rates and power settings that may not be adjusted. Consequently, bit transmission rates and power of transmission are mandated which are optimum only for a single type of equipment configuration.
Prior art systems rely upon interrogation commands from the central control module which are synchronously transmitted to the remote data acquisition modules, relying solely on the central system clock to control times of sampling.
An object of the present invention, therefore, is to assist a field observer to maximize an efficient use of the recording resources available to him for any particular task. Another object of the invention is to provide the greatest possible quantity of data of the highest possible quality for a given equipment configuration.
Another object of the present invention is a seismic system that may have its bit transmission rate tuned to optimize application of the available cable and other equipment to the seismic task objectives.
A further object of the present invention is to utilize deliberately asynchronous sampling of data at the remote units to increase efficiency of utilization of the network components.
Also an object of the present invention is the provision of a configurable seismic telemetry network having multiple data transmission paths available by remote selection. A further object of the invention is a remotely actuated termination point for data interrogation signals.
An additional object of the present invention is a seismic telemetry network in which all data carriers may function at the same bit transmission rate.
Still another object of the invention is a seismic telemetry network in which data transmission base lines may be operated at transmission rates greater or less than those of receiver lines when advantageous to the survey geometry. Prior art provides base lines operating- at fixed transmission rates higher than the receiver line transmission rates. These prior art systems do not provide means to easily vary the bit rate of base line transmission to take advantage of differing requirements of seismic surveys or to match base line bit rate to the bit rate of the receiver line transmissions.
Other objects of the invention include an extension of receiver line take-out distances by optimizing data signal strength. Transmission electrical power influences the distance over which reliable telemetry can occur with higher power required for longer distances. Prior art does not provide ability to vary power as may be required to optimize communication for variable transmission distances over different cables, such as may be used within a project or on projects with differing requirements. Power conservation is an important consideration in prolonging battery life in a distributed seismic data acquisition system. Conservation of battery power in the distributed telemetry units by limiting transmission power to a minimum required for reliable communication is an object of this invention.
Receiver line take-out distances are also enhanced by an increase in data transmission efficiency. By an optimization of communication for a given receiver line take-out distance, the weight of equipment for a given system configuration is reduced.
Also an object of the invention is an increase in the time density of data transmission by minimizing wasted time between data packets.
A further object of the invention is to increase the efficiency of data telemetry by excluding information from the data packet that would identify the signal processing unit that originated the data and its time of creation (which reduces the amount of data that is to be transmitted) and to use the position of the data packet within the data stream to implicitly communicate data packet identity.
The capacity and option to selectively split the data-reporting route of portions of receiver lines is also an object of the present invention.
Another object of the invention is to provide network elements that are interconnectable and able to perform multiple functions thereby maximizing flexibility and efficiency of equipment utilization.